1. Field of the Invention
This invention relates to the recovery of fluorine, uranium and rare earth metal values from waste liquors produced as a by-product in the production of phosphoric acid by the hydrochloric acid decomposition of tricalcium phosphate minerals.
2. Description of the Prior Art
The production of phosphoric acid by the hydrochloric acid decomposition of tricalcium phosphate minerals is well known in the prior art. In such a process, of which U.S. Pat. No. 2,880,063 (issued in 1959 to Baniel et al.) and U.S. Pat. No. 3,311,450 (issued in 1967 to Alon et al.) are typical, a tricalcium phosphate mineral is digested with hydrochloric acid to form a digest solution which is filtered to remove insolubles containing calcium fluoride and most of the silica from the tricalcium phosphate mineral, thereby producing a filtrate termed the aqueous acidulate liquor, which contains CaCl.sub.2, HCl and H.sub.3 PO.sub.4 in addition to fluorine, uranium and rare earth metal values present in the tricalcium phosphate mineral treated. The acidulate liquor is contacted with a suitable organic solvent to extract H.sub.3 PO.sub.4 into the organic phase, which is subsequently separated from the aqueous phase and treated to recover phosphoric acid therefrom. During the extraction operation, some additional HCl is introduced to maintain a concentration of about 2 weight percent HCl in the aqueous phase throughout the extraction to improve the efficiency of phosphoric acid transfer into the organic solvent.
The aqueous phase obtained following this separation, termed the "brine raffinate," contains essentially all of the fluorine and rare earth metal values and a portion of the uranium values initially present in the acidulate liquor. Most uranium, however, remains in the organic phase containing the H.sub.3 PO.sub.4 and must be recovered therefrom subsequent to the extraction step recited above.
Discarding the brine raffinate thereby produced represents a significant loss of valuable fluorine, uranium and rare earth metal values. Uranium values, for example, upon recovery, concentration and purification are useful in serving as fuel for atomic reactors. Due to the large tonnages of tricalcium phosphate minerals which are annually produced by industry to obtain phosphoric acid, even the small concentrations of fluorine, uranium and rare earth metal values which are contained in these minerals represent a substantial source of these valuable elements in the aggregate.